RE: Time to replace the batteries.
I have twin 6.354's as well and I did the same thing, threw away my 8D batteries but replaced them with two 4D batteries. *I also got rid of the original battery charger and installed a 40 amp Xantrex charger and remote monitor. *My batteries are on their 8th year now, they don't use water and spin those Perkins up like there is no tomorrow. *
The real key to long battery life was the Xantrex charger which floats the battery bank voltages seperately , monitors the batteries without trickle charging them and cycles them every 21 days if left idle.